This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The anaerobic methanogenic archaea (methanogens) form methane as an end product of their metabolism. It has been estimated that methanogens produce approximately 1 billion tons of methane every year. Methane is a potent greenhouse gas, but also represents a potential valuable source of renewable energy. This ?double-edged sword? makes the study of biological methane generation of relevance to two of the most critical challenges we face in the World today. Found in all methanogens, methyl-coenzyme M reductase (MCR) catalyzes the final step in biological methane production. Methyl-coenzyme M (methyl-SCoM, 2-(methylthio)ethanesulfonate) and coenzyme B (CoBSH, N-7-mercaptoheptanoylthreonine phosphate) are the substrates for the reaction, in which CoBSH serves as the electron donor for the 2 electron reduction of methyl-SCoM to produce methane and a CoBS-SCoM mixed disulfide. MCR contains coenzyme F430, a redox active nickel tetrahydrocorphin that is the most reduced tetrapyrrole known in nature. The large reduction potential of coenzyme F430 means that stabilizing the reduced and active form of the enzyme for structural studies has proved impossible up to this point. Crystallographic studies thus far have been conducted with oxidized catalytically inactive forms. In collaboration with Steve Ragsdale (University of Michigan Medical School) we now believe we can generate the active form of the enzyme, MCRred1, at ~70% occupancy in single crystals through maintaining anaerobic conditions from microbe grow-up to crystal freezing. With these crystals in hand, a plethora of freeze trapped intermediates and inhibited forms become accessible. The high diffraction quality of the crystals (up to 1.2 Angstroms resolution) allows us to understand how structure relates to function in this highly unusual enzyme.